A variety of systems have been proposed which call for the ability to locate an object. For example, in asset control, it would be desirable to locate objects (e.g., laptop computers) within the confines of an office building. It is also desirable to be able to locate a cellular phone user for emergency service purposes.
The Global Positioning System (GPS) is an example of a prior art system for determining the location of an object. While GPS can be extremely accurate in performing location, a disadvantage of GPS is that in urban canyons or within buildings, signals from GPS satellites may be occluded. Additionally, GPS location circuitry can be space consuming and expensive.
One approach taken to address this issue is to use a group of receivers at fixed locations to locate objects containing transmitters. The fixed receivers are connected to a central computer that serves to determine the location of each object based on signals received from one or more of the group of receivers. While this technique is fairly cheap and easy to implement, it can be very computationally complex and does not provide a very accurate location estimate when compared to other location techniques. As is evident, there exists a tradeoff between accurate location techniques that are expensive to implement, and less expensive techniques that are less accurate. It would be beneficial, therefore, to improve on the less-expensive approach described above so that a more accurate/less complex determination of location can be made. Any such technique employed should also be capable of operating in ad-hoc networks, where nodes may be separated by more than one “hop”. Therefore a need exists for a method and apparatus for locating a device that is relatively inexpensive yet, provides accurate location estimates.